Thyrotoxic osteoporosis, which is accompanied by a high fracture risk, is known to arise from pro-resorptive effects of high thyroid hormone. We reported that TSH-signaling deficient mice lacking the TSH receptor (TSHR) display severe osteoporosis, suggesting that low TSH levels also contribute to thyrotoxic bone loss. That haploinsufficient euthyroid TSHR mice had an equally profound phenotype suggested that the effects of TSH were independent of thyroid hormones. Nonetheless, it remains unclear whether TSHR activation by stimulating antibodies in Graves' disease reduces the hyperthyroid bone loss that is due to high thyroid hormones and low TSH. We further showed that TSHR activation inhibits osteoclast formation, function and survival, as well as the production of TNFa. When TNFa is ablated from TSHR-/- osteoclasts the enhanced osteoclastogenesis and osteopenia are both rescued, suggesting that TNFa plays a key role in thyrotoxic bone disease. Recently, we observed that TSH, when injected intermittently as far apart as once every two weeks, prevented and restored ovariectomy-induced bone loss by inhibiting bone resorption and stimulating bone formation. We hypothesize that TSH preserves the skeleton through potent anti-resorptive and anabolic actions, and that a loss of these actions contributes to the bone loss of hyperthyroidism. We will use genetically modified mice and state-of-the-art molecular approaches to understand the role of TSH in hyperthyroid bone loss. We will first attempt to rescue the TSHR-/- phenotype by deleting TNFa or its receptors, p55 or p75, in double mutants, or by transgenically reconstituting TSHRs in TSHR-/- osteoclasts or osteoblasts. Next, we will determine whether stimulating anti-TSHR antibodies given by injection or produced in vivo by adeno-TSHR immunization attenuate hyperthyroid bone loss. Finally, using mice in which TSHRs are restored cell-selectively in osteoclasts or osteoblasts on a TSHR-/- background, we will examine which cell contributes to the prevention and restoration of post-ovariectomy bone loss by TSH. These foundation studies should allow us to consider skeletal protection by TSH in post-menopausal women whose TSH levels are suppressed by thyroxine therapy for non-cancer causes.